Aluminum oxide, gamma-aluminum oxynitride (ALON), and TiN are well known ceramics for applications such as electronic substrates, optical windows, and crucibles. Al.sub.2 O.sub.3 has been used for abrasive grit as well. ALON has been disclosed in U.S. Pat. No. 4,241,000 as an abrasive grit.
The mechanical properties of ceramic materials have been improved in recent years as a better understanding has been gained as to the effects of processing on the final microstructure. It is well known that low levels of porosity and a fine grain size are required for optimal mechanical performance. Microstructures containing both of these characteristics are not readily obtained because as temperatures are increased to promote the elimination of pores during sintering, grain growth is also accelerated. One technique used to overcome this difficulty has been the combination of different crystalline components to form a composite material.
European Patent No. 0,107,571 (French counterpart 821,957 and English language abstract) describes composite ceramics of Al.sub.2 O.sub.3 and ALON which are useful as cutting tools, dies, crucibles, etc. The mechanical properties compared with prior art alumina are said to be improved.
Composites of TiN and Al.sub.2 O.sub.3 have been disclosed in U.S. Pat. Nos. 3,652,304; 4,022,584; 4,204,873; 4,249,914; 4,325,710; 4,366,254; and JP No. 50-89410 (abstract); JP No. 57-16954 (abstract). The utility disclosed is primarily as cutting tools, but in one case as crucibles. In some of these patents additional components were added to modify the performance or sintering behavior.
Composite cutting tools of TiN with Al.sub.2 O.sub.3 and ALON are disclosed in U.S. Pat. No. 4,320,203.
The background art in the Al.sub.2 O.sub.3 /ALON/TiN system is product oriented to relatively large shapes and forms, for example cutting tools, electronic substrates and crucibles, rather than small particulate products such as abrasive grit. The prior art methods of production employ ceramic powders which are pressed or otherwise shaped to the desired form, and then sintered or reaction-sintered to densify. The manufacture of individual abrasive grit by these techniques is impractical due to the size and numbers required. Also, crushing and screening of larger articles to the desired size range is impractical because of the strength and toughness of these materials. Another problem with the prior art process is the high cost of sinterable AlN and TiN powders used in these processes. For example, commercial AlN powders (which are used to form ALON by reaction with Al.sub.2 O.sub.3) typically cost $55-65/kg, and cannot be sintered without extensive milling and size classification. Powders which are readily sinterable may cost as much as $325/kg.
It is known in the patent literature and technical publications to use sol-gel processes for the preparation of spherical, nuclear fuel particles of the carbides and nitrides of uranium and thorium. Typically hydrous sols of uranium oxide and thorium oxide were co-dispersed with carbon, formed into spheres, then gelled and reaction-sintered to form a carbide or nitride sphere Examples of this teaching include U.S. Pat. Nos. 3,171,715; 3,331,783; 3,860,691; and 3,904,736. The final products were typically less than 95% dense.
The preparation of abrasives comprising alumina and other metal oxides by a sol-gel process is disclosed in U.S. Pat. No. 4,314,827.
The use of sol-gel processes to prepare particles from mixed sols of alumina/carbon, alumina/titania/carbon, or alumina/nitrides, followed by dehydration and reaction-sintering to form dense ceramics in the Al.sub.2 O.sub.3 /ALON/TiN system is believed not disclosed in the literature. It is believed to be novel in the art to use materials in the system Al.sub.2 O.sub.3 /ALON/TiN as abrasives.